The delivery of radio frequency (RF) energy to target regions within solid tissue is known for a variety of purposes of particular interest to the present invention. In one particular application, RF energy may be delivered to diseased regions (e.g., tumors) for the purpose of ablating predictable volumes of tissue with minimal patient trauma.
RF ablation of tumors is currently performed using one of two core technologies. The first technology uses a single needle electrode, which when attached to a RF generator, emits RF energy from an exposed, uninsulated portion of the electrode. The second technology utilizes multiple needle electrodes, which have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. U.S. Pat. No. 6,379,353 discloses such a probe, referred to as a LeVeen Needle Electrode™, which comprises a cannula and an electrode deployment member reciprocatably mounted within the delivery cannula to alternately deploy an electrode array from the cannula and retract the electrode array within the cannula. Using either of the two technologies, the energy that is conveyed from the electrode(s) translates into ion agitation, which is converted into heat and induces cellular death via coagulation necrosis. The ablation probes of both technologies are typically designed to be percutaneously introduced into a patient in order to ablate the target tissue.
In the design of such ablation probes, which may be applicable to either of the two technologies, RF energy is often delivered to an electrode located on a distal end of the probe's shaft via the shaft itself. This delivery of RF energy requires the probe to be electrically insulated to prevent undesirable ablation of healthy tissue. In the case of a single needle electrode, all but the distal tip of the electrode is coated with an electrically insulative material in order to focus the RF energy at the target tissue located adjacent the distal tip of the probe. In the case of a LeVeen Needle Electrode™, RF energy is conveyed to the needle electrodes through the inner electrode deployment member, and the outer cannula is coated with the electrically insulative material to prevent RF energy from being transversely conveyed from the inner electrode deployment member along the length of the probe.
The procedure for using the ablation probe requires the insulative coating to have sufficient durability. To illustrate, when designing RF ablation probes, it is desirable to make the profile of the probe shaft as small as possible, namely to have a smaller gauge size, in order to minimize any pain and tissue trauma resulting from the percutaneous insertion of the probe into the patient. Thus, it is advantageous for the electrically insulative material applied to the probes be as thin as possible. However, RF ablation probes are often introduced through other tightly toleranced devices that may compromise the integrity of the thinly layered insulation, thereby inadvertently exposing healthy tissue to RF energy.
For example, probe guides are often used to point ablation probes towards the target tissue within a patient. A typical probe guide takes the form of a rigid cylindrical shaft (about 1-2 inches in length) that is affixed relative to and outside of a patient, and includes a lumen through which the ablation probe is delivered to the target tissue. To maximize the accuracy of the probe alignment, it is desirable that the guide lumen through which the probe is introduced be about the same size as the outer diameter of the probe, thereby creating a tight tolerance between the probe and the probe guide. As another example, ablation probes are also often used with co-access assemblies that allow several different devices, such as ablation probes, biopsy stylets, and drug delivery devices, to be serially exchanged through a single delivery cannula. To minimize pain and tissue trauma, it is desirable that the profile of the delivery cannula be as small as possible. To achieve this, the lumen of the delivery cannula will typically be the same size as the outer diameter of the ablation probe, thereby creating a tight tolerance between the probe and the delivery cannula.
As a result, during the initial introduction of the probe through a delivery device, such as a probe guide or cannula of a co-access system, it is possible that a portion of the insulation may shear off as the probe is introduced through the delivery device. Consequently, the attending physician will either have to replace the probe with a new one or risk ablating healthy tissue. Thus, the durability of the insulative coating is critical to prevent damaging healthy tissue and/or having to discard the probe.
Besides providing the insulation on the ablation probe with the necessary durability, it is also necessary to ensure that the distal end of the ablation probe, where the RF energy will be directed, is in contact with the target tissue. This may be achieved with an imaging device located outside the patient's body, such as an ultrasound imager. The echogenicity of the probe determines how well the probe may be located using ultrasound techniques. That is, the more echogenetic the ablation probe, the easier it is to determine the location of the probe with ultrasound imaging and to ensure accurate contact with the target tissue.
To achieve greater echogenicity, it is known in the art, for example, to make marks or nicks along the shaft in order to increase the amount of edges and surfaces on the shaft, thereby creating a non-uniform surface profile. Echogenicity increases as the number of edges and surfaces for reflecting the ultrasound is increased. This technique may also be applied to insulative coating on the probe shaft. It is also known in the art to have air bubbles interspersed throughout the insulative coating in order to increase echgenicity. However, the inclusion of air bubbles may degrade the integrity of the insulative coating, which may also occur when marks or nicks are made in the insulative coating.
Therefore, there is a need in the art for an ablation probe with an insulative coating having improved echogenicity for properly positioning the ablation device relative to the target tissue, while also having sufficient durability and size to remain intact during insertion and use of the ablation probe.